S

Migration

Migration

Coalescence

Migration

Migration

Migration

Coalescence

Coalescence

0.268 "" Migration 0.117 T Coalescence

Present

Figure 4.19 Genealogies for six lineages initially divided evenly between two demes when the migration rate is low (a) and when the migration rate is high (b). When migration is unlikely, coalescent events within demes tend to result in a single lineage within all demes before any migration events take place. There is then a long wait until a migration event places both demes in one deme where they can coalesce. When migration is likely, lineages regularly move between the demes, and lineages originally in the same deme are as likely to coalesce as lineages initially in different demes. These two genealogies are examples and substantial variation in coalescence times is expected. In (a) M = 4Nem = 0.2 and in (b) M = 4Nem = 2.0. The two genealogies are not drawn to the same scale. MRCA, most recent common ancestor.

for the average time to coalescence for two lineages in the same deme:

time to coalescence for two lineages in different demes this then

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